Diseases caused by dominant, gain-of-function gene mutations develop in heterozygotes bearing one mutant and one wild type copy of the gene. Some of the best-known diseases of this class are common neurodegenerative diseases, including Alzheimer's disease, Huntington's disease, Parkinson's disease and amyotrophic lateral sclerosis (ALS; “Lou Gehrig's disease”) (Taylor et al., 2002). In these diseases, the exact pathways whereby the mutant proteins cause cell degeneration are not clear, but the origin of the cellular toxicity is known to be the mutant protein.
Mutations in SOD1 cause motor neuron degeneration that leads to ALS, because the mutant protein has acquired some toxic property (Cleveland et al., 2001). Neither the nature of this toxic property nor the downstream pathway that leads to the eventual motor neuron degeneration is understood. In mice, only expression of the mutant SOD1, but not elimination of SOD1 by gene knockout, causes ALS. Nonetheless, the gene knockout mice develop numerous abnormalities including reduced fertility (Matzuk et al., 1990), motor axonopathy (Shefner et al., 1999), age-associated loss of cochlear hair cells (McFadden et al., 2001) and neuromuscular junction synapses (Flood et al., 1999), and enhanced susceptibility to a variety of noxious assaults, such as excitotoxicity, ischemia, neurotoxins and irradiation, on the CNS and other systems (Matz et al., 2000; Kondo et al., 1997; Kawase et al., 1999; Behndig et al., 2001).
More than 100 mutations in SOD1 cause ALS. The severity of the disease is correlated with accumulation of mutant protein. Previous research has demonstrated that the level of mutant SOD1 can be specifically lowered using RNAi targeting the mutation sites. The selective inhibition of mutant SOD1 but not wild-type SOD1 expression avoids the adverse effects resulting from inhibition of the wild type SOD1 expression. Treatment of ALS patients with SOD1 mutations by targeting each mutation site may have certain complexities. For example, a large number of vectors may be needed to target each mutant. In addition, mutation sites may vary in terms of RNAi efficiency and specificity. Improved therapies for treating this disease, as well as other gain-of-function diseases, efficiently and selectively block the expression of the mutant protein while retaining expression of the wild type.